Sterile soil is a must-have for your plants to ensure they don’t get any pest, disease, or weed problems. For this reason many gardeners buy new soil each time they pot their plants. But, if you are an earth conscious and or frugal gardener, you’ll eventually end up Googling “Is there a way to reuse your old potting soil?”. At this point you’ll run into one of the most popular methods spread online for reusing soil; sterilizing old potting soil with boiling water. So, does this method actually work?
As a whole, the high heat from boiling water does kill off weed seeds, insects, and pathogens present. However, it’s not a reliable, thorough method. The temperature does not stay consistently high enough to allow uniform permeability throughout the soil compared to the steaming process.
Below, I discuss more on this method along with better alternatives and valuable tips:
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Why Do People Use Boiling Water to Sterilize Soil?
Compared to using chemical options, boiling water seems the most eco-friendly and easy way of sterilizing garden or potting soil. In my opinion, the logic of this is pretty straightforward. We often use hot water to sanitize baby bottles and dishes to get rid of germs on top of washing them. So why not use it on soil as well?
In fact, hot water soil sterilization is not something new.
There was an experiment carried out way back in the 1970s in Japan to test the validity of this method. The consensus is that it does work but not without certain drawbacks. However, it’s worth noting that this was done on soil in fields and Rockwool potting medium containing root rot pathogens.
But as I mentioned in the introduction, it’s not exactly foolproof. It may not be as too-good-to-be-true as the freebie award you get for being the supposed 100th visitor to a site or store but you might feel a bit duped after reading this. There are several reasons why the boiling method is not that practical:
- There are heat-resistant pathogens that can survive at sustained temperatures of 212°F/100°C. Anything above that is more likely to kill them, which is impossible to achieve with just boiling water.
- The temperature will rapidly drop as soon as it’s applied to the soil. This defeats the purpose of sterilizing because the heat needs to stay at 212°F/100°C for at least 30 minutes to kill all the microorganisms present.
- The heat will only vaporize pathogens and pests on the surface level. This is a problem for garden soil because the same buggers can still thrive and hide deeper in the ground, waiting to come back up once the heat dies down. As for potting soil, it’s a bit easier. The trick is to spread it out evenly into a thin layer of about 2-4 inches before applying boiling water. But that is also dependent on how much soil you’re sterilizing.
So the real question is: how much effort are you willing to put into sterilizing your old soil with boiling water? It seems doable if it’s a small amount of potting medium. But large batches or on a garden patch itself may not be practical. If this is just too much work for you, buy new, sterile soil (Amazon link) for your plants instead. Or use the soil in your back yard if you plan on growing in ground.
Alternatively, you can start a food garden table if the ground is just not viable to work with. If you’re still adamant about sterilizing your soil because it seems like a darn waste to throw it out, hop onto the next section for alternative methods.
Should I pour boiling water on potting soil?
As a general rule, sterilize old potting soil with boiling water only if it had pest or disease problems before. This is useful for killing off any lingering eggs or larvae and pathogens. Otherwise, it’s an unnecessary step.
What Are Alternative Soil Sterilization Methods?
Believe it or not, there are other ways to sterilize your soil that are done in large-scale agriculture productions even to this day. Here we will focus on how you can take inspiration from them and try to replicate it in your home gardening. Here are two of those methods:
What is it? It’s a process of using the sun’s heat in summer to bring the soil’s temperature up within a layer of plastic. The hot steam rising off the damp soil from the heat is trapped, allowing it to sterilize the surrounding soil. This is primarily used to kill off weed seeds but is also helpful in eliminating pests and pathogens on the surface level.
How long does it take? About 4-6 weeks. It may even take longer for areas that don’t get a lot of sun.
How do I do it?
- Purchase a clear, 2-4mm thick plastic liner (Amazon link). It’s important you get a transparent sheet to allow as much sun to pass through and heat the soil. As for the liner’s thickness, it’s up to your discretion but it has to be durable enough to last several weeks, be it from the heat or curious pets and critters. In the link I’ve provided above, I deliberately chose a clear, 4mm thick liner to help you get a head start if you’re not sure where to get it in physical stores.
- Pick out an area in your garden space that receives daily sunlight without shade.
- Lay out about 5×3 feet of the liner on the ground. Depending on the amount of soil you’ll be sterilizing, the dimensions will vary, so feel free to adjust!
- Remove any chunks and leftover debris like roots or twigs in the soil and give it thorough watering to dampen it.
- Spread a layer of moist soil evenly on the liner, making sure it’s about 2-4 inches thick. Note: Do not compress the soil down too much to prevent compaction.
- Leave a 5-6 inches gap between the sides of the liner and the soil.
- Put another layer of transparent plastic over the top, ensuring it’s smooth and taut across the soil.
- Secure the liner with heavy objects such as bricks or bags at the sides to prevent it from flying away during windy days.
- Check on your soil weekly to observe if water droplets are still present underneath the liner. If there aren’t any, you’ll have to water the soil again.
Some have suggested replicating the same steps using plastic baggies instead if you have limited space. But unless you’re willing to do much more work and use more plastic than necessary, you’re better off trying the above method.
How do you naturally sterilize soil?
Solarization is a natural way of sterilizing soil using the sun. Covering a damp garden patch with a clear plastic liner allows heat to build up within and produce steam, effectively killing off microorganisms in the ground. However, this only works well up to 12 inches of the soil. It doesn’t generally affect the lower depths where some pests and pathogens can hide away from the heat.
2. Steaming the soil
What is it? It’s a method of utilizing steam coming off boiling water to sterilize the soil. This method works because the heat sustains at a high temperature, coating every part of the soil. Most nurseries use a specific machine called an autoclave to achieve this. In large fields, a steam generator is used instead for maximum efficiency.
How long does it take? About 30 minutes. The duration may vary depending on the equipment used and the amount of soil.
How do I do it?
- Spread the soil thinly into each steamer rack/basket, about 3-4 inches thick.
- Place it over boiling water and cover for 30 minutes.
- Afterward, switch the steamer off and leave the soil for another 15 minutes before taking it off to cool down completely.
I highly recommend setting a specific steamer aside to sterilize your soil. You don’t want to reuse the same one when cooking your food. On that note, your kitchen will also stink up for a while after doing this (this is why I personally just buy new soil).
Some home gardeners also use an oven or a microwave to sterilize their soil. They apply the same principles of using heat to torch the microbes in the potting media before reusing it for new plantings. I don’t personally endorse this, mainly because it’ll stink up a perfectly good oven and microwave.
Helpful Tips for Sterilizing Soil at Home
If you’re really dead set on sterilizing your soil at home, here are a few tips to keep in mind:
- Use a heat-proof thermometer (Amazon link) to monitor the soil’s temperature to ensure it doesn’t go over 212°F/100°C. This will cause organic compounds in the soil to break down and release harmful toxins. It’s highly likely in potting media with leftover fertilizers, be it organic or synthetic. If the temperature does go over the boiling point, age the soil for a few days before using. If you’re not willing to sacrifice your meat thermometer for this (which you really shouldn’t use for cooking food AND sterilizing soil), you can have a look and buy a new one in the link above.
- Add compost or slow-release fertilizers (Amazon link) to replenish the nutrients in the soil after sterilization. The high heat kills off both harmful and beneficial microorganisms. This is not a problem in garden soil because live microbes will multiply rapidly again anyway. In a sterile potting media, this is not the case. So you’ll have to give the soil and your plant a helping hand to make sure it gets the nutrients it needs. The fertilizer I recommend in the link is quite effective but if you’re not keen on using the synthetic kind, you can use worm castings (Amazon link) instead. It’s one of my favorite natural slow release fertilizers/composts – impossible to over fertilize with and it has a beneficial microbiome already inside of it. No crazy chemicals in this stuff to worry about, creating a healthy, biodiverse, fertile soil. Your plants will love it. If you want to learn more, you can check out my article on my visit to a worm farm here!
- Sterilizing your potting media is unnecessary, especially if the previous plant was not infected or infested. Where possible, skip this method and just start planting. It’s often safe to reuse old potting soil as is, provided you add new nutrients to it.
Sterilizing soil may not be for everyone. But hopefully, this sheds some light on the concept so you can decide whether it’s for you. Otherwise, purchasing a new bag of soil may just be the simple answer you need, at least, it was for me. Happy planting!
AGRIOS, GEORGE N. (2005). Plant Pathology || CONTROL OF PLANT DISEASES. , (), 293–353. doi.org/10.1016/B978-0-08-047378-9.50015-4